The present invention generally relates to masks and charged particle beam exposure methods using the masks, and more particularly to a mask having transmission hole which is partially irradiated by a charged particle beam when exposing variable patterns corresponding to elements of a circuit pattern of an integrated circuit, and to a charged particle beam exposure method which uses such a mask for the charged particle beam exposure.
The photolithography technique was popularly used to form fine patterns. But as the integration density of integrated circuit increased, charged particle beam exposure methods using an electron beam, an ion beam and the like or, an exposure method using an X-ray have been developed to form even finer patterns.
According to the electron beam exposure method which uses the electron beam, it is possible to form fine patterns of 1 .mu.m or less because the spot of the electron beam can be reduced to several .ANG.. However, because the electron beam exposure method draws the pattern, the spot of the electron beam must be reduced as the pattern size is reduced, and the exposure time becomes considerably long. The block exposure method was developed to overcome this problem of long exposure time.
The block exposure method uses a mask having a plurality of grouped regions, and each grouped region includes a plurality of blocks. Each block includes a plurality of holes which correspond to the patterns which frequently appear in the circuit pattern such as the integrated circuit (IC). The charged particle beam is irradiated on a selected one of the blocks.
FIG. 1 shows an essential part of an example of a conventional exposure apparatus for carrying out the conventional block exposure method. An electron beam output from an electron gun 1 is formed into a beam B1 having a rectangular cross section as it passes through a rectangular transmission hole 2. The beam B1 is deflected towards a specified transmission hole of a transmission mask 4 by deflection electrodes 3. The transmission mask 4 includes a plate with a plurality of grouped regions 4a as shown in FIG. 2A, and each grouped region 4a is made up of a plurality of blocks 4b.
As shown in FIG. 2B, each block 4b within the grouped region 4a of the plate includes a plurality of transmission holes 5 having shapes corresponding to elements which form the circuit pattern such as the IC. The block 4b may include only one transmission hole corresponding to a single pattern which is used for exposing variable shapes or, transmission holes corresponding to alignment patterns. Each block 4b is exposed during one exposure by the beam B1.
The deflection electrodes 3 electrostatically deflect the beam B1 to expose one selected block 4b within one grouped region 4a. But when selecting other blocks 4b within other grouped regions 4a, the transmission mask 4 must be moved mechanically by mask moving means Ax and Ay. The mask moving means Ax moves the transmission mask 4 in the direction of the X-axis, and the mask moving means Ay moves the transmission mask 4 in the direction of the Y-axis.
A beam B2 which is formed by passing the beam B1 through the transmission hole 5 of the selected block 4b is projected on a wafer 8 via a reduction lens 6 and an objective lens 7. The beam B2 is projected onto a predetermined position on the wafer 8 by an electrostatic deflector 9 and an electromagnetic deflector 10.
In order to effectively reduce the number of exposures and improve the throughput using this block exposure method, it is conceivable to make the area of the block 4b which becomes the exposure repeating unit as large as possible. But in this case, it also becomes necessary to increase the cross section of the electron beam. If the cross section of the electron beam is increased, the charge density of the electron beam decreases, and it becomes necessary to increase the irradiation time of each beam exposure. As a result, there is a limit to improving the throughput by increasing the area of the block 4b.
On the other hand, it is also conceivable to increase the area of the transmission mask 4 itself, so as to increase the number of blocks 4b which can be arranged on the transmission mask 4. But in this case, the number of times the transmission mask 4 must be moved mechanically by the mask moving means Ax and Ay increases. As a result, the throughput does not improve notably, and in addition, it becomes necessary to improve the function of deflecting the electron beam.
In view of the above, an improved exposure method was previously proposed in a U.S. Pat. No. 5,036,209. This proposed method uses the transmission mask 4 having the blocks 4b grouped within the respective grouped regions 4a as shown in FIG. 2A. In addition, the grouped regions 4a and the blocks 4b within each grouped region 4a are respectively arranged so that the holes 5 and the blocks 4b which are time-sequentially used for the exposure are located close to each other and in such a manner that the time required to move the transmission mask 4 mechanically and the time required to deflect the electron beam are minimized.
However, this proposed method is effective only when repeatedly drawing basic patterns. This proposed method is not effective when drawing non-repeating patterns by passing the beam B1 having the rectangular cross section through a part of the transmission hole in the transmission mask 4 for forming variable shapes, as will be described hereinafter.
FIGS. 3A through 3D are diagrams for explaining a method of exposing a desired pattern corresponding to the circuit pattern of the IC, by irradiating the beam B1 having the rectangular cross section over a transmission hole 11 which is used to form variable shapes. The shape of the exposed pattern is varied by controlling the irradiating position of the beam B1 relative to the transmission hole 11. For example, a triangular hole formed in one block 4b is used as the transmission hole 11. The cross section of the beam B1 is varied depending on the overlap of the beam B1 and the transmission hole 11.
In the case shown in FIG. 3A where the beam B1 overlaps a part of the triangular transmission hole 11, a shaped beam B2 having a small triangular cross section is obtained as shown in FIG. 3B. On the other hand, if the beam B1 overlaps the entire triangular transmission hole 11 as shown in FIG. 3C, a shaped beam B2 having a large triangular cross section is obtained as shown in FIG. 3D. However, in the case shown in FIG. 3A, the beam B1 overlaps both the block 4b1 in which the transmission hole 11 is provided and the block 4b2 which is adjacent to the block 4b1. For this reason, no transmission hole can be provided within the adjacent block 4b2 so as to prevent an unwanted pattern from being formed when the beam B1 overlaps the two mutually adjacent blocks 4b1 and 4b2.
Therefore, with respect to one transmission hole 11 within one block 4b which is used to form variable shapes, it becomes necessary to provide at least one adjacent block 4b in which no transmission hole is provided. In addition, depending on the shape of the transmission hole 11 which is used to form the variable shapes and depending on the kind of overlap of the beam B1 relative to the transmission hole 11, it may be necessary to provide more than one block 4b in which no transmission hole may be provided with respect to one block 4b in which the transmission hole 11 for forming the variable shape is provided. In other words, the number of blocks 4b in which no transmission hole may be provided, that is, wasted area, increases. As a result, there is a problem in that the efficiency with which the transmission holes are arranged in the transmission mask 4 deteriorates. Furthermore, if the efficiency with which the transmission holes are arranged in the transmission mask 4 deteriorates, there are also problems in that the time required to move the transmission mask 4 mechanically and the time required to deflect the electron beam increase.